1. Field of the Invention
The invention relates in general to a near-instantaneously companded audio multiplex (NICAM) system and a symbol rate conversion (SRC) method thereof, and more particularly to a NICAM system and a SRC method capable of preventing the overflow and the underrun from happening.
2. Description of the Related Art
A NICAM system is a television broadcast system released by the British Broadcasting Corporation (BBS) and is adopted by so many countries. FIG. 1 (Prior Art) is a schematic illustration showing a conventional NICAM system. The NICAM system 100 includes square root raised cosine (SRRC) filters 102 and 104, a symbol timing recovery unit 106, a differential quadrature phase shift keying (DQPSK) decoder 108, a NICAM deframer 110, a first-in-first-out (FIFO) buffer 112, a digital loop filter 114 and a voltage controlled crystal oscillator (VCXO) 116.
The NICAM system 100 receives an audio signal outputted from a transmitter. The audio signal includes an in-phase signal I and a quadrature signal Q. The symbol timing recovery unit 106 samples the in-phase signal I and the quadrature signal Q, and then the DQPSK decoder 108 decodes the sampled signals to obtain a data signal Data and a strobe signal S. The NICAM deframer 110 decodes to generate multiple symbols according to the data signal Data and the strobe signal S at each timing and temporarily stores the symbols to the FIFO buffer 112. Then, the FIFO buffer 112 outputs the symbols at a local timing rate.
The rate at which the NICAM deframer 110 decodes to generate the symbols is actually a transmission timing rate of the transmitter, which may be different from the local timing rate at an output terminal of the FIFO buffer 112. If the transmission timing rate is higher than the local timing rate, the FIFO buffer 112 may generate the overflow. If the transmission timing rate is lower than the local timing rate, the FIFO buffer 112 may generate the underrun.
Thus, the conventional NICAM system 100 makes the VCXO 116 generate a local clock LC the same as the local timing rate through the symbol timing recovery unit 106 and the digital loop filter 114, and adjusts the rate at which the NICAM deframer 110 decodes to generate the symbols according to the local clock LC so that the rate is equal to the local timing rate. However, the VCXO 116 occupies a larger area, thereby increasing the cost. In addition, the audio signal outputted from the transmitter is an analog signal, and the symbol outputted from the FIFO buffer 112 is a digital signal. Consequently, the VCXO 116 has to match the transmission timing rate of the analog signal with the local timing rate of the digital signal in a complicated manner.